Radiation therapy using high intensity x-rays or particles to destroy cancerous tissue has been in use for several decades. When treating cancer, it is usually important to irradiate only a precisely defined volume conforming to the tumor, while avoiding irradiation of surrounding tissue. Multi-leaf collimators (MLCs), such as described in U.S. Pat. No. 4,868,843, issued Sep. 19, 1989, to Nunan, (the disclosure of which is incorporated by reference), have been widely adopted because they facilitate shaping of the radiation beam to conform to the site being treated, i.e., the leaves are adjusted so that the beam conforms to the shape of the tumor from the angle of irradiation. MLCs were first used to perform three-dimensional conformal radiation therapy (3D-CRT), wherein the MLC is adjusted to shape the beam to conform to the target from each treatment angle. MLCs have more recently been used to perform “Intensity Modulated Radiotherapy” (IMRT), which allows control over the radiation doses delivered to specific portions of the site being treated. In particular, IMRT allows the intensity distribution of the radiation reaching the patient to have almost any arbitrary distribution. IMRT can be implemented by iteratively positioning the leaves of the MLC, which form an aperture through which radiation is delivered, to provide desired field shapes which collectively deliver the desired dose distribution. IMRT techniques can either be static (“point and shoot” or “move and shoot”), in the sense that the leaves do not move when the beam is on or, alternatively, can be implemented by moving the leaves of the MLC continuously when the beam is on, using a “sliding window” approach. In sliding window IMRT the overall speed of leaf motion and the separation of leaf pairs are independently adjusted as the window moves, such that different portions of the treatment field are irradiated with different doses of radiation through an aperture that changes shape as it is being moved. Recently “arc therapy,” wherein the system gantry moves as radiation is delivered through an MLC, has been adopted as an important mode of treatment. In arc therapy, the leaves of the MLC are adjusted as the gantry revolves around the patient.
Overall, the trend for all of these treatment techniques has been toward much greater precision in delivering a controlled dose of radiation to the target while avoiding healthy tissue. This has made it possible to deliver higher radiation doses to the target over shorter time spans. However, the ability to deliver more radiation more precisely requires the use of better techniques to ensure that the target volume is exactly in the correct position while the radiation is being delivered. Accordingly, small movements of the target have become of greater concern.
Radiation therapy is generally implemented in accordance with a treatment plan which is developed taking into account the prescribed dose of radiation to be delivered to the tumor, as well as the maximum dose of radiation which can be delivered to surrounding tissue. Treatment planning for IMRT and arc therapy is particularly challenging, and sophisticated treatment planning software and algorithms have been developed for treatement planning. Various algorithms for solving the “inverse” problem of translating the prescribed radiation doses and constraints into a delivery plan are well known. Preferably, the computer system and software used to develop the treatment plan provides an output that can be used to directly control the radiation therapy system, including the MLC leaf and gantry movements.
Typically, the desired dose prescribed in a treatment plan is delivered over several sessions, called fractions. Since the treatment volume may change between the delivery of fractions—for example, organs may shrink, swell, or change position—it is often necessary to reimage the treatment volume and to adjust the treatment plan between fractions to accommodate changes. Apart from changes that occur between fractions, tumors and surrounding tissue, including critical organs, may move while a site is being irradiated. Many of these motions occur in a substantially regular, predictable fashion, for example, as a result of normal respiratory motion.
Obtaining the desired biological response in the target region depends upon delivery of the intended fractional dose, thus achieving the planned dose distribution is critical to success of the treatment. While patients undergoing treatment are precisely positioned and immobilized according to well-known techniques, movement in the treatment field can have a significant impact on the effectiveness of a treatment plan. A treatment plan that does not take such movement into account may result too much or too little radiation reaching the intended target region and/or too much radiation reaching surrounding tissue. In the worst case scenario involving IMRT, the target may receive several times the prescribed dose when the target movement is in phase with the MLC aperture movement. On the other hand, if movement of the target region is out of phase with the MLC movement, the tumor may receive a lower than prescribed dose. In practice, interplay between the movement of the IMRT window and the treatment has been reported to generate differences of greater than 10% between the delivered and the planned dose distributions for a single fraction.
One technique for dealing with target movement is disclosed in U.S. Pat. No. 7,221,733, the disclosure of which is incorporated by reference. The '733 patent describes methods and apparatuses for tracking movement of the target and adjusting and/or gating the beam to account for target movements. Gating refers to turning the beam off in response to target movement so that healthly tissue is not irradiated. Another approach to dealing with target movement is disclosed in recently filed and co-assigned U.S. patent application Ser. No. 12/196,639, now U.S. Pat. No. 7,796,731, the disclosure of which is also incorporated by reference. This application teaches techniques for developing treatment plans using leaf sequences that minimize the effects of target movement. Plans made in accordance with the teachings of the '639 application are more robust insofar as they are less sensitive to target movements. There is a continued need, however, for improved techniques for dealing with the moving target problem, as described herein.